As background and with reference to FIGS. 1A and 1B, a representative differential carrier 100 is shown, being the part of a vehicle powertrain responsible for transmitting drive power from the vehicle engine to the vehicle drive wheels. The depicted embodiment of a differential carrier 100 is for a rear axle differential. However, this should not be taken as limiting, as the skilled artisan is well aware of variations in differential type and design.
The differential carrier 100 includes a gear assembly (indicated generically by reference numeral 102) driven by an input drive shaft 104. Gear assembly 102 is operatively connected to a pair of output drive shafts 106, 106′, whereby torque and rotation are transmitted from a vehicle engine 108 to the vehicle wheels 112 (see FIG. 1B). Of course, additional elements are typically included for transmitting drive power (FIG. 1B, see arrows) from the engine 108 to the wheels 112, such as a torque converter 114, a transmission 116, etc.
Returning to FIG. 1A, a hypoid ring gear and differential assembly 118 is operatively connected to the input drive shaft 104. The hypoid ring gear and differential assembly 118 includes a hypoid ring gear 122 which meshes with a hypoid pinion gear 119 of the differential assembly. As is known, a spiral or hypoid gear is so named for its helically-shaped spiral bevel gear teeth, which produce less vibration and noise than conventional straight-cut or spur-cut gears with straight teeth. As shown, an axis of the hypoid ring gear and differential assembly 118/input drive shaft 104 is substantially perpendicular to an axis of the differential carrier 100/output drive shafts 106, 106′. The ring gear 122 is attached to a portion of a differential case 120, which as is known is a housing for the differential carrier 100, in a configuration providing a required meshing between the ring gear 122 and the hypoid pinion gear 119. Gear assembly 102 may also include a side gear 126. As the input drive shaft 104 rotates, so does the hypoid pinion gear 119, driving rotation of the ring gear 122. By this rotation, torque and rotation are transmitted via output drive shafts 106, 106′ to wheels 112.
Alignment of these components during assembly of an axle or rear drive module is important, since as explained the ring gear 122 must mesh with the hypoid pinion gear 119 in a completed axle or rear drive module assembly to transmit the needed torque/rotation to the vehicle wheels 112. Typically these elements are fabricated of different and potentially weld-incompatible materials. For example, a ring gear 122 is often fabricated of steel or an alloy which may or may not be carburized, and if carburized (case-hardened) may have a carbon content of >0.8%. A differential case 120 is often fabricated of high-carbon materials such as nodular ductile iron, and may have a carbon content of 2% or more. Welding such dissimilar materials is challenging at least due to the different material melting temperatures, as well as the resultant high carbon content of the weld interface which may result in weld cracking. For this reason, the most common method for attaching the two is to provide a bolt-on connection using conventional fasteners. While effective, such conventional attachment means increase the required labor and attendant cost, and also contribute to undesirable increases in weight and packaging size.
For this reason, despite the above-mentioned challenges various welding techniques have been considered as an alternative to conventional fasteners to attach the ring gear to the differential case. Laser welding has been attempted to provide a strong attachment despite the incompatibility of the materials of which the hypoid ring gear and differential case are fabricated. In laser welding, typically a nickel feed wire is used to provide a strong and consistent weld in spite of the above-mentioned incompatibility in materials and high-carbon weld surfaces which can crack. Disadvantageously, the high weld temperatures and rapid cooling rates associated with laser welding can cause a drive ring gear to become distorted or warped, preventing the required precise alignment between the ring gear and the mating pinion gear. Moreover, conventional laser welding techniques produce weld spatter that may bond onto the teeth of the ring gear and other components of the vehicle differential assembly, potentially resulting in wear and reduced lifespan and/or failure of the componentry. Likewise this condition may manifest itself as an undesirable Noise/Vibration/Harshness result in the final axle/vehicle assembly. To avoid such weld spatter resulting from laser welding, it is necessary to provide shielding and to implement post-welding maintenance and cleaning protocols. Moreover, laser welding is highly energy-inefficient compared to other welding techniques and requires specialized safety and maintenance protocols due to the use of laser technology. Still more, components to be laser welded must be extremely clean, most commonly laser-cleaned. Each of these factors undesirably adds to labor requirements and attendant costs of manufacturing/assembling a hypoid ring gear/differential case assembly.
Thus, a need is identified in the art for improvements to processes for joining ring gears to differential cases during vehicle drivetrain/powertrain manufacture and assembly. Such improvements should take into account joining materials having significantly different carbon contents, and should provide a weld interface that is low in carbon content despite such dissimilarities in the carbon content of the materials being joined.